Coatings may be prepared in the gas phase (using chemical vapor deposition or physical vapor deposition—See, e.g., Silicon Processing for the VLSI Era, S. Wolf and R. Tauber, Lattice Press Calif., USA, 2000—Chapters 6 and 11) or in the liquid phase. When a coating is prepared in the liquid phase, the coating liquid used in the process may contain a variety of components, such as binders, solvents, and additives. (See, e.g., Modern Coating and Drying Technology, E. Cohen and E. Gutoff, Wiley VCH, 1992, pp 1, which is hereby incorporated by reference in its entirety.) The coating liquid may also be identical in composition as the final coated film, such as in case of molten materials.
Various methods exist for applying the coating liquid to the substrate, including dip coating, rod coating, knife coating, blade coating, air knife coating, gravure coating, roll coating, slot coating, slide coating, curtain coating, Langmuir Blodgett coating, spray coating, spin coating, and the like. (See, e.g., Modern Coating and Drying Technology, E. Cohen and E. Gutoff, Wiley VCH, 1992, pp 6-10.) In many of these methods (but not all), the substrate is brought into contact with the coating liquid and a relative motion between the substrate and the coating liquid is induced.
These liquid coating processes rely on a balance between: a viscous force; a body force, such as centrifugal force, gravity, and/or friction (depending on the particular configuration); pressure of (gaseous) ambient air, gas and/or vapor; and surface tension. The magnitude of these forces influences a thickness of the liquid film left behind (see FIG. 1).
An example of a liquid coating process includes pulling substrates (such as plates, foil, or wire) through a bath of molten coating material (where the substrate, wire, or foil material has typically a higher melting point than the coating substance). Other examples include wire coating with polymers, metal coating with Zn film from molten Zn baths, dip coating, and the like.
Various methods exist to control the coating film thickness. Some of the methods that are used to control the coating film thickness include the use of “air knives,” metering blades, knives, metering rolls, and gravure rolls. (See, e.g., Modern Coating and Drying Technology, E. Cohen and E. Gutoff, Wiley VCH, 1992, pp 2-3.) Examples include the following. Air knives consist of slit-shaped nozzles through which pressurized gas (or air) is blown at high speed. The impact of the gas stream on the liquid surface reduces the thickness of the film left behind, through the impact force. An inherent side effect of these air knives is that they typically involve strong flows of gases which may lead to additional contamination from the air flow and strong evaporation effects, leading to additional contamination (due to enhanced solvent evaporation). Metering blades (also known as “doctor blades”) or knives reduce the film thickness through scraping away excess liquid from the substrate. Metering rolls and gravure rolls determine the amount of coating liquid that is delivered to the moving substrate.
Ramdane and Quéré report in “Thickening factor in Marangoni coating” (Langmuir 1997) on solutions containing surfactants that create a Marangoni flow, which thicken the film when pulling a solid out of solution. However, this method is not applicable for reducing the film thickness to be comparable to a film thickness resulting from pulling a solid out of pure liquid.
Fanton et al. teach in “Thickness and shape of films driven by Marangoni flow” (Langmuir 1996) that the film thickness is dependent on a surface tension gradient, which can be induced by a temperature gradient. However, this method is not applicable for all kinds of substrates.